It is well known in the art of acoustics that electrical energy may be converted into acoustic energy and vice versa by the use of transducers. Two commonly known transducers are an electrodynamic transducer and a piezoelectric transducer. In electrodynamic transducers, an alternating electric current passes through a coil so as to interact with a steady radial magnetic flux causing the coil to vibrate. Accordingly, the coil drives a diaphragm which radiates sound waves from one side when an opposite side is enclosed. This transduction process is reversible when sound waves strike the diaphragm to set up a periodic variation of air pressure adjacent to the diaphragm causing it to vibrate. As the moving coil disturbs the magnetic flux, an electromagnetic field is generated which causes a current to flow when a load is connected to the coil terminals. A transducer is also created when electrical energy is applied to materials that have piezoelectric properties such as ceramic or quartz. As is well known, piezoelectric materials expand when electrical energy is applied thereto. Likewise, the piezoelectric material contracts when the electrical energy is removed. As such, if a rapidly alternating electric current is applied to the piezoelectric material, it expands and contracts accordingly. Therefore, the piezoelectric materials respond with a vigorous resonant vibration.
The aforementioned electrodynamic and piezoelectric transducers are used to produce intense underwater acoustic signals. However, these transducers have several shortcomings. For example, the magnetic circuit and wire coil of the electrodynamic transducer is inefficient in converting electrical energy to acoustic energy. The piezoelectric materials are quite stiff such that a large mass is required to attain a transducer which resonates at low frequencies. Other methods of providing low frequency resonators do not have the structural integrity to withstand the environmental surroundings in which they must operate.
To surmount the short-comings of piezoelectric transducers it is known to use flexural bars or plates attached to ceramic piezoelectric materials such that when the ceramic material expands from an applied current, the flexural bars or plates are driven into a low frequency bending mode. A low resonant frequency is obtained by interconnecting multiple bars or plates together in a mechanical series to increase the displacement thereof. Unfortunately, this type of transducer is limited because ceramic piezoelectric material is weak in tension. One proposed solution for overcoming this weakness is by inserting the ceramic piezoelectric material within a spring which amplifies the motion of the material. Furthermore, these springs are combinable either in series to obtain greater displacement or in parallel to obtain a greater force. However, this proposed solution still does not have an especially large surface area to provide a high amplitude low frequency underwater acoustic transducer.
Another proposed solution is to construct an underwater acoustic projector composed of individual ceramic plates configured in a ring. The ceramic plates in the ring are electrically connected in parallel such that when an alternating voltage is applied to the ceramic plates, a radially alternating ring displacement results in acoustic radiation. Large radial displacements are required to radiate low frequency acoustic energy. One drawback of this proposed solution is that piezoelectric ceramic materials are limited in the voltage amplitude that can be applied thereto. Thus, for an underwater acoustic projector of a given size, the maximum acoustic energy that can be radiated is limited by the properties of the piezoelectric ceramics used. As discussed earlier, since the piezoelectric ceramic material is quite stiff, the low frequency ceramic ring transducers must have a large diameter to achieve a low frequency resonance.
It is clear that there is a need in the art for an efficient, high amplitude low frequency acoustic transducer. Furthermore, there is a need in the art for such a high amplitude low frequency acoustic transducer that maximizes the expansion and contraction of ceramic piezoelectric materials, while reducing the amount of tension applied to such materials to achieve the desired result.